Shadows and rays on a screen

Shadows and rays on a screen

Demonstration

Shadows form on a screen when objects interrupt the light from a lamp.

Apparatus and materials

• Compact light source (quartz iodine lamp) or 100 W 12 V lamp

• L.T. variable voltage supply (12 V 8 A)

• Retort stand and boss

• White screen or blank wall

• Card with slot

Technical notes

If you don't have a compact light source use a 48 W 12 V lamp.
Safety

Be aware that compact light sources using tungsten-halogen lamps without filters are significant sources of UV. Ensure that no-one can look directly at the lamps.

Procedure

a Set up the compact light source in front of the screen or wall, preferably with the room darkened.

b Put various obstacles between the source and the screen so that shadows are seen. A card with a slot will give straight shadows on the wall. The shadows will be sharpest when the edges of the card and the slot are parallel to the lamp filament.

c Move the light source closer to the screen so that a wide beam of illumination falls across it.



d Bring up a card with a slit in it as illustrated to show how a 'ray' of light can be made.

Teaching notes

1 The demonstration in b shows what happens when a lamp throws light onto an object and a shadow falls onto a screen.

2 You can vary the relative distances between the lamp and object, and the screen and object, to show the umbra and penumbra in the shadow. You might want to mention eclipses.

3 The narrow slit placed in the beam in d shows how a ray of light can be produced. The ray of light appears to travel in a straight line.